This proposal presents an extensive program of spectroscopic studies of iron and zinc model complexes of heme and chlorin prosthetic groups and of several such iron proteins with magnetic circular dichroism (MCD) spectroscopy and of physical, structural and mechanistic studies of an oxygen-activating heme iron enzyme, secondary amine mono-oxygenase (SAMO). The major objectives follow. (1) By investigating structurally defined bio-mimetic ligand complexes of iron and chlorin systems, similar zinc complexes and iron proteins, the utility of MCD spectroscopy as a probe of the electronic structure, and ultimately the axial ligand identity, of iron heme and chlorin components in proteins will be ascertained. We will examine (a) the influence of ferric heme iron spin state and coordination number on MCD intensity, (b) the effect of various bio-mimetic ligands (X) on the MCD properties of six-coordinate X-ferrous-thiolate and X-ferrous-CO heme complexes as models for protein states of that composition, particularly hemoprotein H-450 and SAMO, (c) ferric and ferrous chlorin complexes with bio-mimetic ligands and iron chlorin-substituted myoglobin, horseradish peroxidase and cytochrome P-450 in order to establish MCD spectral fingerprints for ligand sets in chlorin-containing proteins, and (d) the MCD spectra of three iron proteins thought to contain a chlorin prosthetic group so as to better define their coordination structures. (2) By studying the physical and mechanistic properties of SAMO, we will further define the structural and functional requirements for heme iron mono-oxygenase enzymes and by examining the spectroscopic properties of chloroperoxidase (CPO), we intend to better define its active site structure and heme iron environment. Specifically, we will determine (a) the subunit composition, location of substrate binding sites, several key mechanistic parameters and additional modes of reactivity of SAMO and will relate these physical and mechanistic properties to those of catalytically similar heme enzymes, (b) the active site structure of SAMO through extensive MCD and EPR spectroscopic examination, (c) additional spectral and physical properties of oxy-CPO to facilitate comparisons with known oxygenated heme proteins, and (d) if an active site histidine exists for binding to the heme iron of CPO at low temperature. Taken together, this program of studies will substantially increase our knowledge of the way in which molecular oxygen is activated by heme iron enzymes and will further demonstrate the utility of MCD spectroscopy as a probe of iron structure in biologically important heme and chlorin containing proteins.